The narrow directional microphone is also called a gun microphone because it has a slender acoustic tube (interference tube) the front end of which is open, having an acoustic resistance hole in the peripheral surface thereof. One example of the narrow directional microphone is explained with reference to FIGS. 3 and 4. FIG. 4 is an enlarged sectional view of a unit fixing portion shown in FIG. 3.
As shown in FIG. 3, the narrow directional microphone includes, as a basic configuration, an acoustic tube 10, a microphone unit 20, and a microphone casing 30.
The acoustic tube 10 is a cylindrical body formed of a metallic material such as aluminum, and a front end part 10a thereof is open. Also, sound holes 11 for introducing acoustic waves are formed in the peripheral surface of the acoustic tube 10. In this example, the sound hole 11 consists of a slit hole formed along the axis line direction of the acoustic tube 10, and, although not shown in the figure, is covered with an acoustic resistance material consisting of a nonwoven fabric or a net body.
The microphone unit 20 is a unidirectional condenser microphone unit, and houses an electrostatic acousto-electric converter. As shown in FIG. 4, the acousto-electric converter is configured so that a diaphragm and a backplate (both not shown) are arranged opposedly via a separator in a cylindrical unit casing 21 that has a front acoustic terminal 22 and a rear acoustic terminal 23 and is formed of a metallic material such as aluminum. From the rear portion of the unit casing 21, a signal output terminal 24 is pulled out.
The microphone casing 30 is a cylindrical body that is longer than the acoustic tube 10, has an inside diameter larger than the outside diameter of the acoustic tube 10, and is formed of a metallic material. The microphone casing 30 is put around the acoustic tube 10. If the acoustic tube 10 is taken as an inner tube, the microphone casing 30 is an outer tube thereof.
To a front end part 30a of the microphone casing 30, a front cap having a guard net or the like is attached, and in a portion on the peripheral surface of the microphone casing 30, facing the sound holes 11 of the acoustic tube 10, a large number of openings 31 each having a thin rectangular shape in this example are formed.
A cylindrical part 30b of the microphone casing 30 extending to the rear side beyond the acoustic tube 10 is a portion that is used as a grip. In the cylindrical part 30b, a circuit board 51 having a sound signal output circuit for the microphone unit 20 is housed.
Also, in the rear end part of the cylindrical part 30b, there is mounted an output connector 52 that is electrically connected to a phantom power source (not shown) via a balanced two-core shielded cable (not shown). The microphone casing 30 is formed with machine screw insertion holes 32a for fastening the output connector 52 with machine screws.
Referring to FIG. 4, the microphone unit 20 is arranged in a rear end part 10b of the acoustic tube 10 in such a manner that the front acoustic terminal 22 thereof is directed to the front end 10a side of the acoustic tube 10. For this purpose, a unit holding member 40 is used.
The unit holding member 40 includes a threadedly engaging part having internal threads 41 threadedly engaging with external threads 21a formed at the outer periphery of the unit casing 21, and a member base 42 having an outside diameter smaller than the inside diameter of the microphone casing 30 and projecting from the rear end of the acoustic tube 10. The member base 42 is fastened with a machine screw inserted through a machine screw insertion hole 32b formed in the microphone casing 30.
In the narrow directional microphone configured as described above, as described in Patent Document 1 (Japanese Patent No. 2562295), to reduce wind noise and proximity effect, a gap A serving as a low-pass filter for causing the rear acoustic terminal 23 and the front acoustic terminal 22 to communicate with each other is preferably provided between the unit casing 21 and the acoustic tube 10.
From a viewpoint of acoustic performance, the gap A is required to have a uniform width throughout the entire periphery of the unit casing 21. However, to position the microphone unit 20 coaxially with the acoustic tube 10 without producing eccentricity and tilt, a considerably high-level assembling technique is required.
Also, the unit casing 21 and the acoustic tube 10 are electrically connected to each other via the unit holding member 40, and the unit holding member 40 is fastened to the microphone casing 30 with machine screws, so that electrical continuity between these parts is difficult to secure, and electrostatic shielding is insufficient.
For this reason, if, for example, a cellular phone is used at close range and strong electromagnetic waves radiated from the cellular phone are applied to the microphone, a high-frequency current flows into the microphone unit 20, which sometimes results in the generation of noise.
Accordingly, an object of the present invention is to provide a narrow directional microphone that can surely position a microphone unit coaxially in an acoustic tube without requiring a high-level assembling technique and merely by using a low-cost and easy-to-handle part, and is also excellent in electrostatic shielding.